%ICONURL{"toggleopen"}% gets you the image path: https://hannover.freifunk.net/wiki/pub/System/DocumentGraphics/toggleopen.png
and that will get rendered as:

To display the ICON that is automatically assigned to a given filename pass the full
filename to the %ICON% macro:

Write %ICON{"My wiki documentation.pdf"}% to get

Implementation

Rendering details

Icons are rendered using two different techniques:

Template based rendering. This is the more modern rendering, added by the FamFamFamContrib. The template engine is used to expand a skin template icon:<name_of_icon>. It is only used when the %ICON% macro is called with the icon name. e.g. %ICON{"pencil"}%

Brute force HTLM Code generation. This is the original icon rendering. It is used when a template is not found for the icon, or the %ICON{}% macro is called with a filename or URL.

Rendering of the "file type" icons in the Topic Attachment table.

The Attachment table calls the %ICON% macro using the full filename of the attachment. The ICON macro then maps the file suffix to the icon name
using a mapping table: DocumentGraphics/_filetypes.txt Each line in the table maps a file type extension to an icon name.
In order for the topic attachment table to display an icon, the attachment filetype suffix must be listed in this mapping file.

Customisation

To create a full new set of Icon's, you would need to duplicate this topic,
and then create a set of icon files matching these names as attachments to that copy.
This lengthy process is required to support direct URL usages of the iconset
(its common for users to find out a url to an image, and then paste it into the editor - this 'just' works).

Template customization

If you only want to customise a small set of icons, or add some of your own,
the best way to do this is to create an icon skin template containing your customisations,
and then to add that into your skin path.

eg:

Create a Skin Template for your icons

Either foswiki/templates/icons.mybook.tmpl on the filesystem, or create a topic called MybookSkinIconsTemplate in the System web
(or if the icon modifications are local to a web, the topic can be created in just that web).